1. Field of the Invention
The present invention relates to a particle detector using microchannel plates for detecting charged particles, such as ions and electrons, and also to a mass spectrometer.
2. Description of Related Art
Microchannel plates (MCPs) are used to detect very weak ions and electrons and are important as a detector for mass spectrometry. A microchannel plate is fabricated by forming a multiplicity of holes in a glass plate and making a conductive coating on each surface of the plate. Furthermore, an appropriate substance having a high resistance value is applied on the inner surface of each formed hole.
As a result, if a voltage of about 2 kV is applied between the conductive coatings on both sides of the glass plate, a potential gradient is developed between the opposite ends of each hole. A material having high secondary electron emmissivity is selected as the applied resistive substance, and the internal surface of the hole functions as a dynode.
In this configuration, an accelerating voltage is applied to the surface hit by ions and electrons such that the ions and electrons may enter the inside of each of the multiplicity of holes efficiently. The ions and electrons accelerated and passed into the holes collide against the internal resistive substance, producing secondary electrons. The produced secondary electrons are accelerated and undergo multiple collisions with the resistive substance inside the holes having the potential gradient. As a result, the electrons are multiplied inside the holes and made to leave for the anode from the exit of each hole.
One prior art technique of this kind of apparatus has an anode electrode disposed on the output side of a microchannel plate (MCP). A dielectric body is sandwiched between the anode electrode and a grounding electrode. A signal produced in response to charged particles captured by the anode electrode is conveyed by a coaxial cable. The impedance matching between the anode electrode and the coaxial cable is made by appropriately selecting the thickness of the dielectric body, the relative dielectric constant, or both (see, for example, Japanese Patent Laid-Open No. 2001-273867).
FIG. 1 shows one example of ion detector utilizing the technique disclosed in Japanese Patent Laid-Open No. 2001-273867. The detector has two microchannel plates (MCPs) 1a and 1b which are stacked on top of each other in use. A high voltage (e.g., −4 kV) is applied from an ion acceleration high-voltage source 2 to the ion incident surface of the MCP 1a. 
Furthermore, a high voltage (e.g., about 2 kV) is applied between the ion incident surface of the MCP assembly consisting of the two superimposed MCPs 1a and 1b and the exit surface from which multiplied electrodes exit from an MCP high-voltage source 3 via voltage-dividing resistors R1 and R2. The voltage source 3 produces a high voltage (e.g., 2.1 kV). As a result, the secondary electron emissive surface of the MCP 1b is at a potential of −2 kV. In addition, a potential of 0.1 kV is given to an anode 4 via a resistor R3. Consequently, the anode 4 is at a potential of −1.9 kV.
The electron emissive surface of the MCP 1b is grounded via a capacitor C2 to prevent the potential of the electron emissive surface from varying at the instant when a burst of electrons is emitted from the electron emissive surface of the MCP 1b. In consequence, the time response characteristics of the detector are improved. Hence, distortion in the signal is suppressed to a minimum.
The values of these potentials give examples on the assumption that the detector is used in a mass spectrometer that analyzes the mass-to-charge ratios of positive ions.
The signal responsive to the amount of electrons hitting the anode 4 is passed through a capacitor C1 to cut off the DC component, the capacitor C1 being made up of the anode 4, a dielectric body 5, and an electrode 6. The output signal from the capacitor C1 is supplied to an amplifier 7 operating at ground potential and is amplified.
In this configuration, positively charged ions are accelerated by the negative high voltage applied to the ion incident surface of the MCP 1a and pass into the MCP assembly. The incident ions collide against the inner surface of each hole inside the MCP assembly, producing secondary electrons. The produced secondary electrons further undergo multiple collisions with the inner surfaces of the holes, because a potential gradient is developed inside each hole. As a result, secondary electrons are produced in an avalanche.
In this way, secondary electrons multiplied in proportion to the number of incident ions inside the holes are obtained from the emissive surface of the MCP 1b. The secondary electrons are then accelerated toward the anode 4 that is at a higher potential than the emissive surface of the MCP 1b. The accelerated electrons collide against the anode 4. A signal responsive to the amount of electrons hitting the anode 4 is passed through the capacitor C1 to cut off the DC component, the capacitor C1 being made up of anode 4, dielectric body 5, and electrode 6. The output signal from the capacitor C1 is supplied to the amplifier 7 operating at ground potential, and is amplified.
Under the present circumstances, the anode of the above-described MCP detector can withstand voltages only up to 3 to 4 kV. Furthermore, the diameter of the MCP available today is less than 20 mm. Hence, there is a demand for an MCP detector using larger MCPs, operating at higher speeds, and withstanding higher voltages. Where a capacitor withstanding high voltages is used in a vacuum, if the capacitor is operated at a voltage exceeding 3 kV, field emission occurs from microscopically protruding portions on the surfaces of the electrodes and dielectric body forming the capacitor by the effect of the surrounding triple junction including the capacitor. That is, discharging takes place, making the detector impracticable.
In addition, where an organic polymer of high voltage resistance is used as a capacitor, it can be operated without depending on the size of the MCP or without worrying about the voltage resistance. However, the voltage on the outer fringe of the organic polymer adhesively bonded to the anode becomes unstable. Field emission occurs on the outer fringe of the polymer, resulting in discharging.
A great feature of the detector using MCPs is high speediness of the response signal. To obtain a high-speed signal having low noise, it is necessary to optimize the capacitance of the capacitor C1 of FIG. 1, the capacitance of the capacitor C2, and the inductance depending on the distance from the ground position to the electrode on the side of the MCP 1b through the capacitor C2. Especially, the structure of the detector must be so designed that the route from point A to point B (see FIG. 1) that is the return route for the signal is made shortest. Moreover, the capacitor C2 is required to have the same voltage-withstanding characteristics as the capacitor C1.